1. Field of the Invention
The present invention relates to a positive electrode active material for a lithium ion secondary battery (hereinafter also referred to simply as “positive electrode active material”). More specifically, the present invention relates to a positive electrode active material having excellent cyclic charge-discharge characteristics (cycle characteristics) and excellent high rate characteristics and further to a positive electrode active material having less gas generation in addition to these features.
2. Description of the Related Work
Lithium secondary batteries are widely used as power sources for mobile electronic devices typified by mobile phones and notebook type personal computers. Recently, their application to large capacity power sources such as batteries for electric vehicles is expected.
Currently, various functions are imparted to mobile electronic devices such as mobile phones and accordingly further improvement in high rate characteristics has been required for lithium secondary batteries used as power sources.
In the case, where lithium secondary batteries are used in electric vehicles, service life of 5 years or more is required for the batteries, so that the cycle characteristics of a battery must be improved. Furthermore, in that case, discharging high current becomes necessary, which requires further improvement in high rate characteristics of a battery.
As the positive electrode active materials for lithium ion secondary batteries, mention may be made of a lithium-transition metal composite oxide (LiMO2 (wherein M represents a transition metal element) of a layer crystal structure as typified by lithium cobaltate. As means for improving the cycle characteristics of the positive electrode active material, JP 2000-21402 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”) describes having a sulfate group contained in a lithium-containing composite oxide shown to have a composition expressed by the formula LixM1−yyO2−zXa (wherein M is Co or Ni, N is a transition metal element not identical with M, or at least one element selected from elements of Groups 2, 13 and 14 of a periodic table, X is a halogen, 0.2<x≦1.2, 0≦y≦0.5, 0≦z≦1, 0≦a≦2z) as a positive electrode active material.
However, with the above-mentioned means, it was impossible to satisfy both of the cycle characteristics and high rate characteristics required for lithium ion secondary batteries in recent years.
Furthermore, when a lithium ion secondary battery is left under high temperatures at the time of charging, it may happen that oxygen is released from lithium cobaltate, accompanying transformation or decomposition of the crystals of positive electrode active material. The oxygen oxidizes and burns nonaqueous electrolytic solution to give a significant impact on the safety of the battery. Therefore, further improvement in the thermal stability of positive electrode active material has been desired.